#include <cstdlib>
#include <cmath>
#include <cstdio>
#include <algorithm>
#define max(a, b) (((a) > (b)) ? (a) : (b))
#define min(a, b) (((a) > (b)) ? (b) : (a))
#define sign(x) ((x) > eps ? 1 : ((x) < -eps ? (-1) : (0))) // 符号函数
using namespace std;
const int MAXN = 10000;
const double eps = 1e-8, inf = 1e50;
struct point
{
    double x, y;
    point() {}
    point(double _x, double _y)
    {
        x = _x;
        y = _y;
    }
};
inline double xmult(double x1, double y1, double x2, double y2)
{
    return x1 * y2 - x2 * y1;
}
inline double getarea(point pg[], int n)
{
    double area = 0;
    pg[n] = pg[0];
    for (int i = 0; i < n; i++)
        area += xmult(pg[i].x, pg[i].y, pg[i + 1].x, pg[i + 1].y);
    return fabs(area) / 2.0;
}
point pg[MAXN];
int main()
{
    int n;
    while (scanf("%d", &n), n >= 3)
    {
        for (int i = 0; i < n; i++)
            scanf("%lf%lf", &pg[i].x, &pg[i].y);
        double v;
        scanf("%lf", &v);
        printf("BAR LENGTH: %.2lf\n", v / getarea(pg, n));
    }
    return 0;
}